Suspensions consisting of powdery ceramic material, a solvent or mixture of solvents, one or more than one organic polymeric binders, softeners and dispersing agents are used, for example, to cast thin ceramic green foils which are dried and then subjected to further processing to form electronic components such as, for example, ceramic multilayer capacitors.
A ceramic material of particular interest is barium titanate which is used for the manufacture of ceramic multilayer capacitors because it has a high dielectric constant. In this manufacturing process, in particular, organic solvent systems are used in which the ceramic powder is dispersed and to which organic polymeric binders, softeners and dispersing agents are added to form the casting slip for the foil. To cast thin foils having a thickness &gt;2 .mu.m and exhibiting an improved homogeneity and strength, the aqueous suspensions must consist of very fine-powdered barium titanate having particle sizes smaller than 0.5 .mu.m, preferably in the range from 0.1 to 0.2 .mu.m.
However, besides the average size and the size distribution of the solid primary particles of the ceramic material, also the state of agglomeration of the solid particles in the suspension is also important for attaining a uniform, trouble-free structure of the green foils. For example, it is being recognized more and more that homogeneous, densely packed green foils can only be obtained when the solid particles present in the suspension are completely deagglomerated, individual primary particles, because particle agglomerates and particle aggregates cause the formation of relatively large cavities, bubbles, holes and inclusions which adversely affect the homogeneity and strength of the foils.
A complete deagglomeration of the powder particles is very important, in particular, in the manufacture of very thin foils because in this case the above-mentioned defects caused by particle agglomerates very adversely affect the foil properties. The reason for this being that as the foil thickness decreases, the foil-thickness-related relative dimension of the defect increases. This brings about a deterioration of the homogeneity and strength of the foil.
In addition, complete particle deagglomeration is very difficult, in particular, when very fine materials having an average particle size &lt;0.5 .mu.m are used, because as a result of the small particle diameter and the large surfaces very strong, attractive interactions between the particles take place.
As disclosed in, for example, Japanese Reference No. 62-169480, deagglomeration of very fine solid particles in an aqueous suspension can be attained by subjecting the suspension to ultrasound.
Close examination of such known suspensions reveal, however, that this ideal, deagglomerated condition is not stable for any desired length of time. After a certain time, ranging between 0 minutes and 4 hours, which is governed to a substantial degree by the solid content of the suspension and by the concentration of the polymeric organic binder, the suspensions become unstable, i.e. the particles start to agglomerate.
However, the undesired reagglomeration of the individual primary particles is retarded by suitable dispersing agents. Dispersing agents generate repulsive forces between particles, which forces are in opposition to the attractive forces acting between the particles. It is known that in aqueous systems polyelectrolytes, in particular those having a degree of polymerization in the range from 20 to 100, can very suitably be used for this purpose. Such dispersing agents are adsorbed onto the particle surface and, as a result of their numerous charged functional groups, such as --COO.sup.- or --NH.sub.3.sup.+, cause the particle surface to be charged. The particles which now carry like charges repel one another electrostatically. In general, these electrostatic repulsive forces are much greater than the attractive interactions based on dispersive van der Waals' forces.
The above-mentioned unstable behaviour of the suspensions used for the manufacture of the foils constitutes a major drawback for the practical application of the method on an industrial scale.
As the presence of particle agglomerates adversely affects the properties of the foil, the casting slip should be processed before the onset of agglomeration. However, at characteristic stability times of 0 minutes to 4 hours, this would impose substantial limitations on the steps in the production process. Besides, to ensure constant foil properties in the casting process, the suspensions would have to be manufactured in quantities which can be processed completely into foils within a time range of 0 minutes to 4 hours. However, such a method would be uneconomical and, in addition, would adversely affect the reproducibility of the individual, much to small, batches of suspensions. This means that the suspensions used must be stable for at least 15 to 25 hours. When the loss of time caused by the necessary timely coordination of the various process steps is included, a period of approximately 40 hours in which the state of agglomeration of the suspensions should remain constant, is desirable.